The production of integrated circuits begins with the creation of high-quality semiconductor wafers. During the wafer fabrication process, the wafers may undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. As the size of the integrated circuits continues to decrease and the number of microstructures per integrated circuits increases, the need for precise wafer surfaces become more important. Therefore, between each processing step, it is usually necessary to polish or planarize the surface of the wafer to obtain the flattest surface possible.
For a discussion of chemical mechanical planarization (CMP) process and apparatus, see, for example, Arai, et al., U.S. Pat. No. 4,805,348, issued February, 1989; Arai, et al., U.S. Pat. No. 5,099,614, issued March, 1992; Karlsrud, et al., U.S. Pat. No. 5,329,732, issued July, 1994; Karlsrud et al., U.S. Pat. No. 5,498,196, issued March, 1996; and Karlsrud, et al, U.S. Pat. No. 5,498,199, issued March, 1996.
Such polishing is well known in the art and generally includes attaching one side of the wafer to a flat surface of a wafer carrier or chuck and pressing the other side of the wafer against a flat polishing surface. In general, the polishing surface comprises a polishing pad that has an exposed abrasive surface of, for example, cerium oxide, aluminum oxide, fumed/precipitated silica or other particulate abrasives. Polishing pads can be formed of various materials, as is known in the art, and which is available commercially. Typically, the polishing pad may be a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Scottsdale, Ariz. The hardness and density of the polishing pad depends on the material that is to be polished.
During the polishing or planarization process, the workpiece or wafer is typically pressed against the polishing pad surface while the pad rotates about its vertical axis. In addition, to improve the polishing effectiveness, the wafer may also be rotated about its vertical axis and oscillated back and forth over the surface of the polishing pad. It is well known that polishing pads tend to wear unevenly during the polishing operation, causing surface irregularities to develop on the pad. To ensure consistent and accurate planarization and polishing of all workpieces, these irregularities must be removed.
During the CMP process, workpieces occasionally become dislodged from the workpiece carrier, or they may break during polishing. If a dislodged workpiece or a part of a broken workpiece is allowed to remain on the polishing table, it could contact other workpieces and/or workpiece carriers on the same polishing table and thereby damage or destroy all of the workpieces on the table. Accordingly, it is desirable to detect the presence of a broken or dislodged workpiece immediately and to terminate processing until the situation can be rectified. Typically, this requires a thorough cleaning and/or replacement of the polishing pad, so that workpiece fragments and other debris can be removed so that they do not damage other intact workpieces.
Presently known systems for detecting the loss of workpieces or for detecting broken workpieces are unsatisfactory in several regards. For example, currently known techniques apply a substantially monochromatic light beam, such as, for example, a red LED light source, to the polishing pad surface and a photodetector detects a portion of the applied signal which is reflected from the polishing pad. It is known that the wavelength of light reflected from the polishing pad differs from the wavelength of light reflected from a disk which may pass through the applied monochromatic beam. In accordance with presently known systems, if the wavelength of reflected light changes dramatically, it is presumed that a stray (i.e., lost, dislodged workpiece or workpiece fragment) is detected, and the CMP machine is immediately shut down to prevent further damage to other workpieces on the same table. However, because prior art techniques employ a limited bandwidth light source, such as LEDs, the presence of slurry, deionized water and iodine on the pad and on the wafer itself tend to mask the limited bandwidth light, preventing the light from being properly reflected back to the photodetector. Consequently, many presently known workpiece detection schemes often emit "false" readings whereupon machines are shut down and processing halted even though all disks remain intact within their respective carriers. Similarly, presently known machines often fail to detect a dislodged workpiece if the band of acceptable wavelengths indicative of the polishing pad is broad enough to include the light reflected by a slurry covered workpiece.
A technique for detecting lost or dislodged workpieces on a CMP polishing pad is thus needed which overcomes the shortcomings of the prior art.